Ordinary heating coke, obtained as a secondary product in the manu-
facture of illuminating gas from long-flame bituminous coal, forms more
or less opaque, highly porous lumps of a grey colour. Metallurgical coke,
which is obtained from a fat, short-flame coal, is resistant, sonorous and
pale grey with metallic lustre. In every case, coke consists, apart from
ash, essentially of carbon.
Unwashed coke should contain only 1-2% of moisture ; if washed and air-
dried it may contain as much as 5-6 %. With certain cokes the ash may amount
to 20%, but good samples usually contain 4-10%.
Further, coke usually contains 83-90% C, 0-3-2% H, 2-6% O and 1-2%
N ; the proportion of sulphur may reach 2-5%, but for metallurgical coke the
usual requirement is that it shall not exceed i%. With cokes not excessively
rich in ash, the calorific power varies from 7000 to 8000 calories.
In England it is required that a good metallurgical coke should not contain
more than 4% of water, 8% of ash and 0-5% of sulphur, and that its calorific
power should be about 8000 cals.
These vary in quality with the coal and agglutinating material used
in their manufacture. Bituminous coal is the more commonly employed
and pitch is mostly taken to act as cohesive, although many other materials
have been proposed.
Briquettes are usually brick-shaped, but cylinders, sometimes per-
forated to facilitate access of air, and ovoid forms are also made
With these fuels, in addition to the determinations given under the
heading " General methods/1 it is important to ascertain the cohesion or
compactness. This may be done with a special apparatus, or by a crushing
test normal to the largest face (with brick-shaped briquettes).
In some cases also the resistance to heat is tested, in order to find out
if the briquettes soften when kept for a certain time at a definite tempera-
ture (e.g., 6 hours at 60°).
The determination of the pitch used as agglutinant may be carried out
approximately by extraction in a Soxhlet apparatus with carbon disulphide
until the latter ceases to become coloured (this usually requires at least
10 hours), evaporating the carbon disulphide and weighing the residue
after drying at 120°. In this way, however, only 50-60% of the pitch
actually used is extracted.
The conditions to be satisfied by briquettes vary with the quality and with
the uses to be made of them. In general it is required that they should be homo-
Anzin (Nord) : Caking coal 2 .....